An internal combustion engine is a combustion engine that converts the chemical energy of a fuel into mechanical work by combustion. Combustion takes place in a combustion chamber by the ignition of a mixture of fuel and ambient air. The expansion of the combustion waste gas that is heated by the combustion is used to move a piston.
In particular, internal combustion engines are known from the prior art that use up to 40% of the energy stored in the fuel to move the piston. The remaining energy generated in the course of combustion is—in the case of a cooled engine in addition to the coolant—mostly discharged from the engine in the form of heat by escaping combustion waste gases. To reduce such heat losses, it has therefore been proposed to add a suitable heat recovery system to a generic internal combustion engine. The professional journal “lastauto omnibus 1-2/2013” explains on pages 30 ff the basic construction of corresponding devices known in specialist fields.
Another technical article printed in “MTZ 10|2012” from page 768 uses a device referred to as a heat exchanger, which—in a cyclic process known to a steam specialist as a Rankine cycle—transfers thermal energy from the combustion chamber of the engine to a heat-transfer medium that flows around said combustion chamber. K. Schmiederer, J. Eitel, S. Edwards, E. Pantow, P. Geskes, R. Lutz, M. Mohr, B. Sich, R. Dreisbach, J. Wolkerstorfer, H. Theiβl, S. Krapf and K. Neunteufl, “Use potential of a commercial vehicle engine with a Rankine system, determined by means of power measurements with steady-state and non-steady-state cycles and constant emissions”, 33rd International Vienna Motor Symposium, 2012, studies the reduction in fuel consumption that can be achieved by the proposed approach.
Problematic in this case is the selection of the heat-transfer medium, which should on the one hand have a high boiling point, heat transfer coefficient, heat capacity and thermal conductivity and on the other hand have low viscosity, flammability, toxicity and a low freezing temperature. Heat exchangers known from the prior art therefore use in addition to conventional coolants for example water, ethanol, methanol, ammonia or mixtures based on these liquids as heat-transfer media.
For instance, US 2008/0008605 A1 discloses a high-pressure fuel pump in which the movable part is a metal bellows, which is filled and emptied alternately with hydraulic liquid under high pressure. The amount of fuel in the chamber in which the bellows moves is determined upstream by a solenoid valve, which is controlled by a computer for monitoring the engine. The proposed pump comprises at least two pump units, which are fed by a single inlet common to both units.
FR 2 828 240-A1 also proposes a hydraulic pump for petrol. Each cylinder of the hydraulic pump is connected on one side to a reservoir and on the other side to a check valve, either of which enter into fluid connection under the action of a distribution device controlled by means of a proportional magnet.
DE 103 06 146 A1 describes a low-pressure store for a high-pressure piston pump for pumping fuel, having a liquid-tight, elastic separating structure, which has a first side and a second side, the first side delimiting a part-volume of the low-pressure store, which is hydraulically connected to a low-pressure side of the high-pressure piston pump. The low-pressure store is characterised in that the elastic separating structure delimits the part-volume in a gas-tight manner.
A disadvantage of these conventional heat recovery systems lies in the long-term wear of the movable parts of the heat recovery system, around which flow passes in this manner. In this respect, the liquids mentioned usually have physical characteristic values with regard to density, viscosity, pour point, aniline point, drop point, setting point and neutralising capacity, which make them appear suitable as lubricants only to a limited extent. The chemical compatibility with customary drive components should likewise not be thought unimportant in the case of some of the heat-transfer media used. The tribological loading of the heat recovery system tends to be further increased by the pressure level of the heat-transfer medium necessary to operate the heat exchanger, depending on the configuration.